Origin of γ-rays from young open clusters

The young open cluster Berkeley 87 was predicted to be associated with the COS B γ-ray source 2CG 075+00 on the basis of the experimental evidence of the presence of a shock front around the Wolf - Rayet star ST3 placed in the inner part of the cluster. The CGRO phase-1 data confirm this identification. Protons accelerated at the shock boundary can produce π⁰ via p - p interactions and then γ-rays. With the measured flux F_γ (E > 100 MeV) ≊ 9 x 10^-7 ph cm^-2 s^-1 and the power-law proton spectrum with spectral index γ = 2, the cosmic-ray-energy-density in the inner part (≈ 0.8 pc radius) of Berk 87 is about 100 times greater than that in the vicinity of the Earth. We have calculated γ-ray spectra expected from the decay of π⁰ produced in p - p interactions. The spectra have been normalized to the observed flux of Berk 87 by using different input proton spectra. The comparison of these normalized spectra with the upper limit at 140 TeV, coming from the CASA-MIA experiment, provides constraints on the initial proton spectrum in Berk 87. The calculated γ-ray spectra of Berk 87 are also compared with the sensitivities of some present and future experiments for energies greater than 100 GeV, such as the Whipple and GAMT experiments. We suggest the possibility of measuring cosmic-ray induced lines, such as the 4.44 MeV and 6.13 MeV nuclear de-excitation lines of ¹²C^* and ¹⁶O^* respectively, from Berk 87 with the COMPTEL instrument on board the CGRO. Also the positrons resulting from energetic particle interactions should produce 0.511 MeV annihilation radiation from Berk 87, which could be measured by the OSSE instrument on board the CGRO.